1. Field of the Invention
The present invention relates generally to the field of surge protection, and more particularly to an isolated shield coaxial surge suppressor.
2. Description of the Related Art
Surge protection devices protect electronic equipment from being damaged by large variations in the current and voltage across power and transmission lines resulting from lightning strikes, switching surges, transients, noise, incorrect connections, and other abnormal conditions or malfunctions. Large variations in the power and transmission line currents and voltages can change the operating frequency range of the electronic equipment and can severely damage and/or destroy the electronic equipment. Surge protection devices are typically placed in line with transmission lines and are located near the electronic equipment to be protected. Whenever a surge protection device is connected to a transmission line, it is possible that one end, e.g., a surge port, of the surge protection device is at one ground potential and the other end, e.g., a protected port, of the surge protection device is at another ground potential. This difference in potential can result in the flow of electrical current between the two grounds, which is typically referred to as a “ground loop”. When a signal is transmitted along the transmission lines at various frequencies, such as between about 100 MHz to 6 GHz, and across the surge protection device, interference or distortion caused by the ground loop may alter the signal resulting in what is typically referred to as “hum”. On a video screen, the interference or distortion caused by the ground loop may be seen as dark horizontal “hum bars” across the picture typically moving slowly from the bottom of the picture toward the top. The interference and distortion may modulate the signal trying to pass the surge protection device and may also corrupt the data encoded in the signal, thus making it unusable.
Several devices have been used to eliminate the undesirable effects caused by ground loops, but these devices have primarily focused on cancellation of the interference, rather than prevention. For example, a “hum-stop” or “hum-canceling” coil has been used to produce a cancellation of the low frequency power components on the signal lines. Typically, the hum-canceling coil is a coil of coaxial cable tightly wound around the signal lines.
These types of coils are expensive to manufacture with a wide band frequency response, and invariably produce an attenuation of higher frequencies, due to the long length of the cable contained in the coil. Compensation for this frequency loss requires extra circuitry, either as a pre-emphasis to the signal at the source end of the cable, or as a post-emphasis to the signal at the receiving end of the cable. In either case, there may be some degradation of the signal-to-noise ratio, and the additional circuitry may introduce non-linear distortions that can affect the differential phase or gain, or other characteristics, of the signal path.
Hence, one drawback of conventional surge protection devices is the difficulty in preventing ground loops. Another drawback of conventional surge protection devices is the interference and distortion, such as hum, caused by ground loops. Yet another drawback of conventional surge protection devices is the attenuation of higher frequencies and the degradation of the signal-to-noise ratio.